This invention relates to sieving apparatus.
In GB-A-1462866 there is described such apparatus, for sieving dry particulate material, comprising a base, a frame mounted on the base for vibratory movement with respect thereto, a grating attached to the frame, means for vibrating the frame relative to the base, and ultrasonic means coupled to the grating to effect ultrasonic vibration thereof relative to the frame.
In this known apparatus the ultrasonic means comprises an electro-mechanical transducer, and in particular a magnetostrictive transducer, the body of which is rigidly mounted with respect to the frame and which is coupled to the grating by means of a metal probe.
With such apparatus the low frequency vibration of the grating effected by the oscillation of the frame serves for bulk movement of material on the grating so that all layers of material are presented to the grating, while the high frequency vibration of the grating effected by the ultrasonic means serves to prevent blinding of the apertures of the grating otherwise caused by material adhering to the grating or by particles of the material locking together to bridge the apertures.
A disadvantage of such known apparatus is that the magnetostrictive transducer becomes hot and requires a supply of large volumes of cooling air to maintain a satisfactory temperature during operation. Further, the transducer is relatively large and heavy, and adds significantly to the mass to be oscillated.
In U.S. Pat. No. 4,816,144 there is described such an apparatus in which the ultrasonic means comprises a piezoelectric transducer directly bonded to the grating.
This known apparatus has the advantage that the transducer is relatively small and light, and does not require significant cooling during operation. Further, the transducer is tunable, and can be provided with means for frequency control, and preferably automatic control, whereby the optimum amplitude range of the ultrasonic vibrations of the grating for most effective operation of the apparatus can be achieved and maintained.
The transducer can be energised by a supply circuit including means to sense resonance of the grating and any deviation therefrom, and feedback means operative in response to the output of said sensing means to control the supply to the transducer to maintain resonance of the grating.
A transducer as used in such apparatus has minimum impedance at resonance, and this impedance can be sensed and used to control the output of a free running oscillator by which the transducer is powered.
The impedance can be sensed by sensing the voltage across a resistor connected across the supply to the transducer, the sensed voltage being used as a feedback signal for control of the oscillator.
Although in the known apparatus described in GB-A-1462866 the frame is vibrated relative to the base, this is not essential, it being otherwise possible for the material being sieved to be conveyed to and from the grating by vacuum or pressure differential means, and or by gravity, the vibration of the grating relative to the frame imparted by the transducer being sufficient to effect sieving.